1. Field of the Invention
The present invention is concerned with a very specific improvement to the drill head forming the subject matter of U.S. Pat. No. 4,410,053 granted on Oct. 18, 1983 to Roger MASSE, which drill head is intended to be used for drilling hard material such as rock.
More particularly, the invention is concerned with a new impact hammer assembly for use on such a drill head.
2. Description of the Prior Art
The drill head disclosed and claimed in the above mentioned U.S. Pat. No. 4,410,053 generally comprises a number of impact hammer assemblies distributed about a central support having a like number of flat faces onto each one of which an impact hammer assembly is carried. The assemblies are located around the central support, so that the impact hammers stand at different radial distances from a rotary mounting spindle to which the central support is end-connected. With such a construction, the front of hard material to be drilled is fragmented by impact along concentric, spaced apart, circular paths and by shear between these paths. The head serves, as is known, to drill large holes through rock, in a single pass, by impacting the rock in selected areas to fragment it.
While this drill head is quite efficient, its use presents several drawbacks due to its particular construction.
A major one of these drawbacks is the manner in which the hammers are fixed to the central support. This is done, in the case of each hammer assembly, by means of a pair of pillow-blocks, each involving a half cylindrical seat fixed to the central support, as by welding, and into which seat one end of the cylindrical case of the hammer rests, being clamped therein by a metal strap bolted to the seat and forcing the hammer case against it. Under the severe action of the impact hammers, it will be appreciated that the bolts of the pillow-blocks either rapidly become loose or the straps or seat break, necessitating break-down of the operation for tightening of the bolts or for repair.
Another major drawback of the drill head of U.S. Pat. No. 4,410,053, is that its driver sub-member which receives the drill bit is screwed into the corresponding end of the hammer case. This member becomes loose under the severe blows applied to it by the repeated hammering action of the drill bit, rendering break-down adjustment necessary.
In U.S. Pat. No. 4,410,053, it is also suggested to drive the fragmenting head or drill bit of every impact hammer to make it rotate about its longitudinal axis during the drilling operation. The very purpose of this rotation which is, in practice, positively controlled by a motor contained in the case of each hammer, was allegedly to reduce wearing of the drill bit teeth. After several years of extensive use and testing, it has however been found that such a reduction in wearing of the teeth was not attained, possibly due to the uneven roughness of the rock surface to be drilled by impact.
In order to tentatively overcome or at least reduce the first drawback mentioned hereinabove, it has been suggested by Mr. Roger MASSE, in his U.S. patent application Ser. No. 038,768 filed on Apr. 15, 1987, to use impact hammer assemblies each provided with a one-piece cast case including a connection flange on each side of its longitudinal axis. These flanges which preferably extend the full length of the case, can be bolted to the central support, thereby transmitting the hammer blows directly to the support and thus appreciably reducing any tendency of the bolts to become loose.
To overcome the second drawback mentioned hereinabove and thus avoid loosening of the driver submember, it has also been suggested in the same U.S. patent application Ser. No. 038,768 to use impact hammer assemblies each comprising a one-piece cast case formed with radial connection ears at the "sub end" thereof. Each assembly also comprises a driver sub-member which has radial connection ears operatively engaging over the case connection ears while bolt means releasably secure the drive sub-member and case together through the connection ears. Preferably, the driver sub-member is also a one-piece cast metal member having an axial protrusion which is snugly received into the body housing. The driver sub-member is formed with a through bore coaxial with the body longitudinal axis and opening into the hammer case.
To overcome the third and last deficiency listed hereinabove, it has further been suggested by Mr MASSE in his U.S. patent application Ser. No. 038,768, to use a spline and groove connection between the stem of the bit and its receiving drive sub-member, to prevent any rotation of this bit. This solution has been tested for a while but has not proved to be really satisfactory to improve the resistance property of the bit teeth.